generally "extremely good"
common in UK during mid-70s when it was most often applied to rock music, in which context it would also imply "strong" or "powerful"
the opposite was helluva tame, generally but not necessarily used in a derogatory context.

Barker: "what kind of sounds do you dig, man?
Tender:
"Oh, Zeppelin, Sabbath, Heep .... any hellish beef stuff really"
Barker:
"Yeah me too, Zep are the men - I even like their helluva tame stuff on Zep 3"
Tender: "Wheyyy.... look at that guy with the hellish beef prime Plant locks over there!"
Barker: "Yeah, he looks beef but he's carrying a helluva tame Cat Stevens album under his arm"